An IMS/MMD (Multimedia Domain) network or architecture primarily consists of several signaling entities such as proxy-call session control function (P-CSCF), interrogating-CSCF (I-CSCF), serving-CSCF (S-CSCF), and home subscriber service (HSS) which is usually a database or other repository for user or subscriber information such as authorization data, including information related to services provided to a user. Roaming service and mobility are supported by a combination of Session Initiation Protocol (SIP) components such as the signaling entities, P-CSCF, S-CSCF, I-CSCF, and mobile IP components or nodes, such as home agent (HA) and foreign agent (FA). IMS/MMD architecture mandates that there should be security association (SA) between the mobile and P-CSCF. Secure Internet Protocol (IPSec) is one way of providing SA for signaling and media traffic.
In the MMD, service is not provided until an SA is established between the user equipment (UE) and the network. Typically, UE is a Mobile Node (MN). IMS is essentially an overlay to the packet data subsystem (PDS) and has a low dependency on the PDS as it can be deployed without the multimedia session capability. Consequently, a separate SA is required between the multimedia client and the IMS before access is granted to multimedia services.
The primary focus of the IMS/MMD security architecture is the protection of SIP signaling between the subscriber and the IMS. The IMS defines a means of mutual authentication between the subscriber and the IMS, and also specifies mechanisms for securing inter- and intra-domain communication between IMS network elements.
In an IMS/MMD environment, P-CSCF is the first entry point in a visited network as far as SIP signaling is concerned. A P-CSCF has multiple roles in the network as defined by IMS/MMD standard. Primarily it acts like the first hop outbound proxy for the mobile. Any SIP related messages (e.g., REGISTER, INVITE etc.) have to traverse via this P-CSCF. Although these are supposed to behave as proxies, they are call-stateful proxies, and thus each P-CSCF is equipped with client daemon and server daemon and is capable of generating any non-INVITE messages. Thus during handoff, P-CSCF plays an important role both for signaling and media. Media cannot traverse through a new packet data servicing node (PDSN) in the visited network during handoff until a new SA between P-CSCF and MN has been established. Thus it is essential to have all security states transferred from old P-CSCF to new P-CSCF before any new media passes through the new PDSN for security optimization. For an IMS/MMD architecture, where all P-CSCFs are in the visited network, this has even more significance in terms of local quality of service (QoS) and pricing information. Since P-CSCF maintains such information, until these parameters are properly transferred from the old-P-CSCF to new P-CSCF, the handoff will not be fast. In order to have a seamless handover for a real time session between two visited networks, fast P-CSCF transition is essential, and is commonly known as P-CSCF fast handoff.
How the signaling and media will be affected if there is no fast P-CSCF handoff mechanism in place is described. After that, the fast handoff mechanisms both for proactive and reactive handovers are discussed and details regarding how the signaling and media delay during handover can be minimized are presented.
FIG. 1 gives the details call flow during handoff when P-CSCF fast handoff is absent. In this scenario, normal SIP registration with authentication and key agreement (AKA) happens with the new P-CSCF.
Unless the registration is successful, the gate at the new PDSN will not be open and thereby will not allow any packet from Mobile Node to traverse through the new visited network, except MIP binding update and SIP registration signaling. This is primarily because no SA exists between MN and new P-CSCF. Thus, there can be a substantial delay depending upon the load at the new P-CSCF and the time required to establish an IPSec SA between Mobile Node and new P-CSCF. A boxed portion of FIG. 1 indicates the period whereby the session will be interrupted for signaling, except SIP registration and MIP binding update, since no other signaling messages can be exchanged during this time, e.g., auxiliary signaling such as paging, IM etc. Once the gate at new PDSN is open, MN can send and receive other signaling along with the media.
Similarly, FIG. 2 shows the media delay during handoff without fast P-CSCF handoff mechanism in place. The media delay is significant since MN cannot send any packet unless the gate at the new PDSN is open during the registration process although MIP update is performed earlier. Also, there can be a substantial increase to the delay value depending upon the P-CSCF load, HA load and the distance between HA and MN. This delay can range from several hundred milliseconds to seconds in some cases. Thus for delay sensitive real time applications, delay is an issue.